System Diagnosis

To diagnose trouble in the system, it is useful to know how each sensor influences the pulse length and/or frequency. As you may remember, the air flow sensor is the only sensor whose signal is not a variable voltage. It is instead a pulsating constant voltage of varying FREQUENCY. The higher the air flow, the faster the air flow sensor pulses. Other sensors - throttle position sensor, air and engine temperature sensors, manifold pressure sensor, and throttle position sensor - put out variable voltages. The O2 sensor acts almost like an ON/OFF device, its output fluctuating back and forth between about 0.9V when the mixture is rich and about 0.1V when the mixture is lean.

The illustrations which follow show the output characteristics of the various sensors under normal conditions.

FIGURE 7:

Air Flow Sensor

The air flow sensor output cannot be measured with your voltmeter. An oscilloscope would be the best way to "see" the signal of this sensor. Fig. 7 graphs the frequency range of this signal.

FIGURE 8:

Coolant and Air Temperature Sensors

The coolant sensor and air temperature sensors both have the same characteristics: their output voltages range between about 1/2 V hot and 5V cold. Fig. 8 shows signal voltages and sensor resistances at five different temperatures.

FIGURE 9:

O2 Sensor (Oxygen Sensor)

The O2 sensor sends a voltage signal that is high when the air/fuel ratio is richer than 14.8 to 1 and drops off sharply when the air/fuel ratio becomes leaner than 14.8 to 1. See Fig. 9. Because the computer changes the mixture instantly in response to the O2 sensor signals, the sensor output will normally fluctuate rapidly between 0.9 and 0.1V.

Note: Measurement may be difficult: Some meters cannot respond fast enough to sense the fluctuations, and analog meters have enough internal impedance that they cannot pick up the tiny current output of the O2 sensor.

FIGURE 10:

Pressure Sensor

The manifold pressure sensor voltage output can range from 0V to 4V. When the engine is stopped, the output should be about 2.3V. See Fig. 10.

FIGURE 11:

Throttle Position Sensor

Output voltage of the Throttle Position Sensor should be familiar to every Mitsubishi technician because the initial setting is 0.48V. This is the voltage output of the sensor when the throttle is closed. At wideopen throttle, the output voltage increases to about 5V. See Fig. 11.



SYSTEM DIAGNOSIS

The output graphs on the preceding pages are for sensors in normal operating condition. A sensor whose output is not within the ranges on the graphs is suspect.

To help in diagnosing the ECI system, the following diagnostic chart should be helpful.